Re: [PATCH 3/3] selftests/mm: virtual_address_range: Dump to /dev/null

[Dev Jain <dev.jain@arm.com>]

On 08/01/25 7:00 pm, David Hildenbrand wrote:
> On 08.01.25 07:09, Dev Jain wrote:
>>
>> On 07/01/25 8:44 pm, Thomas Weißschuh wrote:
>>> During the execution of validate_complete_va_space() a lot of memory is
>>> on the VM subsystem. When running on a low memory subsystem an OOM may
>>> be triggered, when writing to the dump file as the filesystem may also
>>> require memory.
>>>
>>> On my test system with 1100MiB physical memory:
>>>
>>>     Tasks state (memory values in pages):
>>>     [  pid  ]   uid  tgid total_vm      rss rss_anon rss_file 
>>> rss_shmem pgtables_bytes swapents oom_score_adj name
>>>     [     57]     0    57 34359215953      695      256 0       439 
>>> 1064390656        0             0 virtual_address
>>>
>>>     Out of memory: Killed process 57 (virtual_address) 
>>> total-vm:137436863812kB, anon-rss:1024kB, file-rss:0kB, 
>>> shmem-rss:1756kB, UID:0 pgtables:1039444kB oom_score_adj:0
>>>     <snip>
>>>     fault_in_iov_iter_readable+0x4a/0xd0
>>>     generic_perform_write+0x9c/0x280
>>>     shmem_file_write_iter+0x86/0x90
>>>     vfs_write+0x29c/0x480
>>>     ksys_write+0x6c/0xe0
>>>     do_syscall_64+0x9e/0x1a0
>>>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> Write the dumped data into /dev/null instead which does not require
>>> additional memory during write(), making the code simpler as a
>>> side-effect.
>>>
>>> Signed-off-by: Thomas Weißschuh<thomas.weissschuh@linutronix.de>
>>> ---
>>>   tools/testing/selftests/mm/virtual_address_range.c | 6 ++----
>>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/virtual_address_range.c 
>>> b/tools/testing/selftests/mm/virtual_address_range.c
>>> index 
>>> 484f82c7b7c871f82a7d9ec6d6c649f2ab1eb0cd..4042fd878acd702d23da2c3293292de33bd48143 
>>> 100644
>>> --- a/tools/testing/selftests/mm/virtual_address_range.c
>>> +++ b/tools/testing/selftests/mm/virtual_address_range.c
>>> @@ -103,10 +103,9 @@ static int validate_complete_va_space(void)
>>>       FILE *file;
>>>       int fd;
>>>   -    fd = open("va_dump", O_CREAT | O_WRONLY, 0600);
>>> -    unlink("va_dump");
>>> +    fd = open("/dev/null", O_WRONLY);
>>>       if (fd < 0) {
>>> -        ksft_test_result_skip("cannot create or open dump file\n");
>>> +        ksft_test_result_skip("cannot create or open /dev/null\n");
>>>           ksft_finished();
>>>       }
> >>   >> @@ -152,7 +151,6 @@ static int validate_complete_va_space(void)
>>>           while (start_addr + hop < end_addr) {
>>>               if (write(fd, (void *)(start_addr + hop), 1) != 1)
>>>                   return 1;
>>> -            lseek(fd, 0, SEEK_SET);
>>>                 hop += MAP_CHUNK_SIZE;
>>>           }
>>>
>>
>> The reason I had not used /dev/null was that write() was succeeding 
>> to /dev/null
>> even from an address not in my VA space. I was puzzled about this 
>> behaviour of
>> /dev/null and I chose to ignore it and just use a real file.
>>
>> To test this behaviour, run the following program:
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #include <fcntl.h>
>> #include <sys/mman.h>
>> intmain()
>> {
>> intfd;
>> fd = open("va_dump", O_CREAT| O_WRONLY, 0600);
>> unlink("va_dump");
>> // fd = open("/dev/null", O_WRONLY);
>> intret = munmap((void*)(1UL<< 30), 100);
>> if(!ret)
>> printf("munmap succeeded\n");
>> intres = write(fd, (void*)(1UL<< 30), 1);
>> if(res == 1)
>> printf("write succeeded\n");
>> return0;
>> }
>> The write will fail as expected, but if you comment out the va_dump
>> lines and use /dev/null, the write will succeed.
>
> What exactly do we want to achieve with the write? Verify that the 
> output of /proc/self/map is reasonable and we can actually resolve a 
> fault / map a page?
>
> Why not access the memory directly+signal handler or using 
> /proc/self/mem, so you can avoid the temp file completely?
>

We want to determine whether an address belongs to our address space. 
The proper way to do that is
to access the memory, get a segfault and jump to signal handler. I 
wanted to avoid this code churn,
so chose to use write() so that I can validate the address without 
getting a segfault.